I used it quite a long time ago and thought it was useful. But as Greg said, you have to start out with a sound that is fairly flat and flexible for transformation. Don't expect miracles as from making water into wine, but possible improvements are within reach with some experimentation.
A poor quality recording will not yield improvement, however, if this is what you have to start out with.
Most importantly, remember that "Microphone Modeler first compensates for the characteristics of the original mic, then imposes the curve of the target microphone." So if your mic is one of those listed, you will have a good starting point for transformation. There will be a drop down menu of microphones availbale for both source and target microphones.